Ultrasonic pulse-echo techniques are in widespread use for the nondestructive evaluation of materials. Since these techniques are sometimes applied in adverse conditions such as elevated temperatures and pressures, it is not practical to contact ultrasonic transducers directly with the materials tested thereby exposing the transducers to the adverse conditions. Instead, acoustic waveguides are installed between the transducers and the materials to transmit acoustic waves from the transducer into the material and back to the transducer for the detection of any discrete defects in the material under testing.
Known in the art is an elastic waveguide (U.S. Pat. No. 4,743,870 issued May 10, 1988 to Jen et al) for propagating acoustic waves which consists of an elongated solid core region and an outer cladding. The bulk longitudinal wave velocity of the cladding is larger than that of the core. Both the cladding and the core acoustic wave velocities are substantially uniform (step profile). The waveguide is useful for the propagation of elastic waves in a longitudinal mode.
Due to the wave diffraction effects and the finite diameter of the waveguide (buffer rod), spurious echoes may be present in the analyzed sample image. Also termed trailing echoes, these echoes will always arrive later than the directly transmitted or reflected longitudinal echoes and often interfere with the desired signals.
One way of dealing with trailing echoes is mentioned in a paper by H. J. McSkimmin, "Measurement of Ultrasonic Wave Velocities and Elastic Moduli for Small Solid Specimens at High Temperatures", J. Acoust. Soc. Am. 31, 287-295 (1959). A screw thread groove can be ground through the length of the rod to suppress spurious pulses (echoes) arising from mode conversion at the cylindrical boundaries of the rod. In the McSkimmin paper, the rod is made of fused silica. C. K. Jen at al. (J. Acoust. Soc. Am. 88 (1), July 1990) tested aluminum rods having two spiral V grooves surrounding the rod in the clockwise and counterclockwise direction. The tests confirmed that the provision of a thread is effective in reducing trailing echoes. Another alternative to the same effect, tested by Jen, (see the above-mentioned Jen paper), was to disturb the rod boundary such that the waves generated due to mode conversion along the rod would not be added in phase at the receiver. Based on this approach, a tapered buffer rod was prepared and found effective in reducing the trailing echoes.
It is also known in the art to produce optical fibers or lenses with graded refractive index profiles. For silica glass based optical applications, graded refractive index (n) profile can be achieved by way of chemical vapor deposition, ion exchange and sol-gel methods. In designing and manufacturing such fibers or lenses, it is essential to adjust the concentration of a dopant in the radial direction while maintaining the concentration uniform in the axial direction.
Recently, a relation between the refractive index profile and the acoustic velocity profile in silica or other materials has been investigated and reported in a paper by C. K. Jen (the present inventor), C. Neron, A. Shang, K. Abe, L. Bonnell, J. Kushibiki and C. Saravanos on "Acoustic Characterization of Optical Fiber Glasses" (SPIE, vol. 1590, pp. 107-119, Boston, OE/Fibers'91, September 1991). The paper presents acoustic characterization of silica glasses doped with GeO.sub.2, P.sub.2 O.sub.5, F, TiO.sub.2, Al.sub.2 O.sub.3 or B.sub.2 O.sub.3. Measurements of acoustic velocity at various dopant concentrations and associated measurements of optical refractive index have shown that alumina as dopant increases the acoustic velocity while the other dopants decrease it compared to that of the pure fused silica. The fiber preforms having step and graded refractive index profiles also show step and graded acoustic velocity profiles respectively.